The WGA workround wit Trixie didnt work for me but I found another way.
If you don't pass WGA validation, download
LegitCheckControl.rar
which should be a RAR file containing the legitcontrolcheck DLL. Make sure IE is not running, then rename the legitcontrolcheck.dll in \windows\system32 and put the new legitcontrolcheck.dll in there. Now you can install IE7. Enjoy.